Power converter having switching elements formed of unipolar devices using a wideband gap semiconductor

ABSTRACT

An inverter circuit (120) is configured so as to perform synchronous rectification by six switching elements (130). The switching element (130) is formed of an unipolar device (SiC MOSFET in this case) using a wideband gap semiconductor. The inverter circuit (120) uses the body diode (131) of SiC MOSFET (130) as a freewheeling diode during synchronous rectification.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of copending application Ser. No.12/865,590, filed on Jul. 30, 2010 which is the National Phase under 35U.S.C. § 371 of International Application No. PCT/JP2009/000205, filedon Jan. 21, 2009, which claims the benefit under 35 U.S.C. § 119(a) toPatent Application No. 2008-021759, filed in Japan on Jan. 31, 2008, allof which are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a power converter having switchingelements formed of unipolar devices using a wideband gap semiconductor.

BACKGROUND ART

As a material for switching elements for a power converter, silicon arewidely used at present. However, the characteristics of switchingelements made of silicon have almost reached their theoretical limits.As the materials that exceed the theoretical limit of silicon, thereexist wideband gap semiconductors such as SiC (silicon carbide), GaN(gallium nitride) and diamond, the development of these being inprogress. Power devices using wideband gap semiconductors have superlow-loss and high-speed high-temperature operation characteristics. Ofthese wideband gap semiconductors, the most remarkable one is the SiCdevice, and SiC MOSFETs are regarded as promising switching for powerconverter.

In inverter circuits and the like that drive inductive loads, a diode isconnected in parallel with a switching element. The diode of this kindis called a freewheeling diode, and functions to flow a reverse current.In an inverter using a SiC MOSFET as a switching element, a study on aconfiguration in which a SiC Schottky barrier diode (hereinafter SiCSBD) is connected in parallel with the SiC MOSFET, and the SiC SBD isused as a freewheeling diode, has been made.

CITATION LIST Patent Document

PATENT DOCUMENT 1: Japanese Patent Publication No. H10-327585

PATENT DOCUMENT 2: Japanese Patent Publication No. 2007-129848

Non-Patent Document

NON-PATENT DOCUMENT 1: Nikkan Kogyo Shimbun Ltd. “Semiconductor SiCTechnology and Application”, written and edited by Hiroyuki Matsunami,2003

NON-PATENT DOCUMENT 2: Ohmsha, Ltd. “Basic and Application of SiCDevices”, edited by Kazuo Arai and Sadafumi Yoshida, 2003

SUMMARY OF THE INVENTION Technical Problem

Provision of the configuration as above makes it possible to reduce lossat the freewheel diode, but this configuration causes a problem of theapparatus becoming bulky and increasing in cost because the SiC SBD isneeded.

Solution to the Problem

A power converter of the present invention is configured so as toperform synchronous rectification by means of a switching element (130),and is characterized in that the switching element (130) is comprised ofa unipolar device using a wideband gap semiconductor, and the body diode(131) inside the unipolar device is used as a freewheeling diode.

The power converter is characterized in that the unipolar device turnson when a reverse current flows through the body diode (131) used as thefreewheeling diode, so that the reverse current flows through theunipolar diode, thereby performing synchronous rectification.

In this way, use of the body diode (131) makes it possible to provide aconfiguration with the switching element (130) only without the need ofproviding a separate freewheeling diode (132), hence reduce cost.Further, execution of synchronous rectification makes it possible tomake the switching element (130) conduct and inhibit conduction lossmore than the body diode (131) alone does.

The power converter is characterized in that the power converter is usedfor an air conditioner. Further, the power converter is characterized inthat the relationship between the effective current value (I_(rms)) andthe on-resistance (R_(on)) of the switching element (130) underintermediate load condition of heating operation of the air conditioner,satisfies: I_(rms)<0.9/R_(on).

When synchronous rectification is performed with the switching elementselected in this way, it is possible to attain an equivalent or greaterefficiency without providing a separate freewheeling diode (132), henceachieve both cost reduction and high efficiency in the intermediate loadcondition of heating operation.

The apparatus is characterized in that any one of SiC, GaN and diamondis used as the wideband gap semiconductor.

The apparatus is characterized in that the unipolar device is a MOSFET.

The power converter is characterized in that, of an inverter (120),converter (110), matrix converter (700) and boosting chopper circuit(111) which are constructed to perform synchronous rectification bymeans of the switching element (130), at least one is included.

Advantages of the Invention

In the power converter of the present invention, since the body diode(131) of switching element (130) is used as a freewheeling diode, it ispossible to reduce cost without the need of providing a separatefreewheeling diode (132). Further, execution of synchronousrectification enables a reverse current to flow through the switchingelement (130) and inhibit conduction loss more than the body diode (131)alone does. When synchronous rectification is carried out by selectingthe switching element so that the relationship between the effectivecurrent value (Inns) and the on-resistance (R_(on)) of the switchingelement (130) under intermediate load condition of heating operation ofthe air conditioner, satisfies: I_(rms)<0.9/R_(on), it is possible toattain an equivalent or greater efficiency without providing a separatefreewheeling diode (132), hence achieve both cost reduction and highefficiency in the intermediate load condition of heating operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a power converteraccording to the first embodiment of the present invention.

FIG. 2 is a diagram showing the basic concept of synchronousrectification.

FIG. 3 is a diagram showing one example of a configuration in which aSiC SBD is connected in parallel with a SiC MOSFET so as to be used as afreewheel diode.

FIG. 4 is a diagram schematically showing the voltage-currentcharacteristics of a SiC MOSFET, the body diode of SiC MOSFET and a SiCSBD (132).

FIG. 5 is a diagram showing a configuration of a power converteraccording to the second embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of a power converteraccording to the third embodiment of the present invention.

FIG. 7 (a) is a diagram showing a configuration of a power converteraccording to the fourth embodiment of the present invention. FIG. 7 (b)is a diagram showing a configuration of the bidirectional switch (710)shown in FIG. 7 (a).

DESCRIPTION OF REFERENCE CHARACTERS

-   -   10 a.c. power source    -   20 motor    -   100, 500, 600 power converter    -   110 converter circuit    -   111 boosting chopper circuit (power factor enhancement circuit)    -   120 inverter circuit    -   130 SiC MOSFET (switching element)    -   131 body diode    -   700 matrix converter

DESCRIPTION OF EMBODIMENTS

Now, the embodiments of the present invention will be described withreference to the drawings. In the drawings, the identical parts orcorresponding parts are allotted with the same reference characters anddescription of those will not be repeated. The description of thepreferred embodiments is a mere example in nature, and is not intendedto limit the scope, applications and use of the invention.

The First Embodiment

FIG. 1 shows a configuration of a power converter according to the firstembodiment of the present invention. This power converter (100)rectifies alternating voltage of an a.c. power supply (10) using aconverter circuit (110) and converts the direct current into athree-phase current by means of an inverter circuit (120) to supply theresultant current to a motor (20). This motor (20) is to drive acompressor that is provided for a refrigerant circuit in an airconditioner. Here, in FIG. 1 the a.c. power supply (10) is assumed tosupply a single-phase current, but may supply a three-phase current.

The inverter circuit (120) is configured such that synchronousrectification is performed by six switching elements (130). Theswitching element (130) is comprised of a unipolar device (SiC MOSFETherein) using a wideband gap semiconductor. The inverter circuit (120)uses the body diode (131) of the SiC MOSFET (130) as a freewheelingdiode when performing synchronous rectification. Here, synchronousrectification is a control method whereby the SiC MOSFET (130) is turnedon when a reverse current flows through the freewheeling diode (131) toflow the reverse current to the MOSFET side, as shown in FIG. 2. Thismakes it possible to reduce conduction loss when a reverse currentflows.

As a conventional technology for performing synchronous rectificationusing the body diode as a freewheeling diode, there exists a techniquein which the body diode of a Si MOSFET is used as a freewheeling diode.However, since the turn-on voltage of the body diode of a Si MOSFET islow (about 0.7V), the body diode instantly turns on even if synchronousrectification is carried out. Accordingly, synchronous rectification haslittle effect. In contrast to this, in the case where the body diode(131) of the SiC MOSFET (130) is used as a freewheeling diode as in thepresent embodiment, since the turn-on voltage of the body diode (131) ofthe SiC MOSFET (130) is high (about 3V), the body diode (131) will notturn on unless the current becomes large when synchronous rectificationis carried out. As a result, when the body diode (131) of the SiC MOSFET(130) is used a freewheeling diode as in the present embodiment,synchronous rectification provides a greater effect than in the casewhere the body diode of a Si MOSFET is used as a freewheeling diode.

When the body diode of the Si MOSFET is used a freewheeling diode so asto achieve synchronous rectification, there is the problem that arecovery current flows due to the body diode. To deal with, some wayssuch as lowering the switching speed to reduce recovery current,manipulating the circuit configuration so that no current will flowthrough the body diode, or reducing the loss due to recovery current byproviding an additional circuit, have been devised (Patent Documents 1and 2).

As described in Background Art, when a SiC MOSFET is used as a switchingelement, a study on the configuration in which a SiC SBD (132) isconnected in parallel with the SiC MOSFET (130) so as to be used as afreewheeling diode has been made, as shown in FIG. 3. In thisconfiguration, since there is a large difference between the turn-onvoltage (about 3V) of the body diode (131) of the SiC MOSFET (130) andthe turn-on voltage (abut 1V) of SiC SBD, it is possible to flow areverse current through only the SiC SBD (132) that is less in recoverycurrent while no reverse current will flow through the body diode (131).It is known that a SiC SBD can markedly reduce recovery current andswitching loss. In this way, SiC MOSFET can easily reduce recoverycurrent as compared to the case of a Si MOSFET. Here, with a Si MOSFET,the turn-on voltage of the diode that can be connected in parallel andused as a freewheeling diode, is as low as that of the body diode of theSi MOSFET, hence it is impossible to prevent any reverse current fromflowing through the body diode.

On the other hand, because the recovery current of a SiC pn diode havingthe same structure as the body diode (131) of the SiC MOSFET (130) issmall, its switching loss is an order of magnitude lower than that of aSi pn diode. Accordingly, it is possible to significantly reduce therecovery current and switching loss in the present embodiment.

As needing the SiC SBD (132), the configuration shown in FIG. 3 poses acost increase problem. However, when the body diode (131) of the SiCMOSFET (130) is used as freewheeling diode so as to effect synchronousrectification as in the present embodiment, it is possible to provide aconfiguration with the SiC MOSFET (130) only without the need ofproviding the SiC SBD (132), hence the cost can be reduced. Further, useof synchronous rectification makes it possible to make the MOSFET sideconductive and reduce the conduction loss compared to the case where thebody diode (131) is used alone. In particular, under light loadconditions, it is possible to inhibit loss compared to the case wherethe SiC SBD (132) is used (this aspect will be described later).

Though in FIG. 1, the configuration for effecting synchronousrectification by using the body diode (131) as a freewheeling diode isapplied to all the six switching elements (130) in the inverter circuit(120), it is possible to apply this to part of switching elements (130)only.

(Switching Element Selecting Conditions)

In the inverter circuit (120) of the present embodiment, the SiC MOSFET(130) turns on by virtue of synchronous rectification. In theconventional configuration (the configuration in which the SiC SBD (132)is connected in parallel with the SiC MOSFET (130) and is used as afreewheeling diode, see FIG. 3), the SiC SBD (132) turns on. Here, FIG.4 schematically shows the voltage-current characteristics of the SiCMOSFET (130), the body diode (131) of the SiC MOSFET (130) and the SiCSBD (132). The SiC MOSFET (130) shows a fixed resistance characteristic.The turn-on voltage of the SiC SBD (132) is about IV and the turn-onvoltage of the body diode (131) of the SiC MOSFET (130) is about 3V. Theturn-on voltages are determined depending on physical properties, andcannot be set arbitrarily.

Comparing the characteristics between the configuration of the presentembodiment and the conventional configuration (FIG. 3), when theterminal voltage is equal to or lower than the turn-on voltage of theSiC SBD (132), the present embodiment is more efficient. However, as thecurrent further increases, the conventional configuration presentshigher efficiency. Accordingly, in an operating mode in which a largecurrent is flowed under the rated conditions or heavy load conditions,the conventional configuration using the SiC SBD (132) presents higherefficiency. On the other hand, under light load conditions, the SiC SBD(132) has little effect so that the configuration of the presentembodiment with the SiC MOSFET (130) alone presents higher efficiency.

Here, the loss of the present embodiment and the configuration (FIG. 3)when current i=(√2)I_(rms) sin θ flows, is represented by the followingformulas 1 to 3.

-   -   The present embodiment        R _(on) ×I _(rms) ²  (Formula 1)    -   The conventional configuration        Vf×(2√2/π)Irms . . . where Vf=const  (Formula 2)        αI _(rms) ²+β(2√2/π)Irms . . . where Vf=αi+β  (Formula 3)

I_(rms) is the effective value of the current, R_(on) is theon-resistance of the SiC MOSFET (130), Vf is the terminal voltage of theSiC SBD (132). (Formula 2) is an approximation when Vf is set at aconstant value. (Formula 3) is an approximation when Vf is approximatedin the first order.

As understood from FIG. 4 and the above formulae, the present embodimentshows a greater loss under the rated condition or under heavy loadconditions but shows a smaller loss under light load conditions. Unliketypical load conditions where the efficiency at the rated load isregarded as important, in air conditioning usage, the operating timeunder light load conditions is longer. Therefore, for effective energysaving, an operation efficiency under light load conditions is demanded.The operating condition having the greatest influence on the effectiveenergy saving within Japan is the condition called the intermediate loadcondition of heating operation in which operation is performed at halfthe rated heating capacity.

When the effective current at the half heating load is the SiC MOSFET(130) is selected so that the conditions meeting the following (formula4) and (formula 5) will hold.I _(rms1)<(2√2/π)Vf/R _(ori) . . . where Vf=const.  (Formula 4)I _(rms1)<(2√2/π)β/R _(on)−α) . . . where Vf(i)=αi+β.  (Formula 5)

When synchronous rectification is performed with the switching elementselected in this way, it is possible to attain an equivalent or greaterefficiency without use of the SiC SBD (132) and hence achieve both costreduction and high efficiency.

Further, taking into account that the turn-on voltage of the SiC SBD(132) is about IV, the above (formula 4) can be simplified as in(formula 6) below when Vf is set at 1V.I _(rms1)<0.9/R _(on).  (Formula 6)

This further simplifies selection of the switching element.

The Second Embodiment

FIG. 5 shows a configuration of a power converter according to thesecond embodiment of the present invention. This power converter (500)performs synchronous rectification, using the body diode (131) of a SiCMOSFET (130) as the diode for a boosting chopper circuit (111) used as apower factor enhancement circuit. With this synchronous rectification,efficiency is improved, especially under light load conditions. Further,since a SiC device is used, the recovery current is made markedly lowcompared to that with a Si device so that it is possible to reduceswitching loss.

The Third Embodiment

FIG. 6 shows a configuration of a power converter according to the thirdembodiment of the present invention. This power converter (600) performssynchronous rectification, using the body diodes (131) in SiC MOSFETs(130) as the rectifying diodes for a converter circuit (110). Though inFIG. 6, a commercial power supply (10) is assumed to be a single-phasealternating current, three-phase alternating current may be used.Further, only part of the rectifying diodes in the converter circuit(110) may use the body diode (131) of the SiC MOSFET (130) while theothers may use usual diodes.

The Fourth Embodiment

FIG. 7 (a) shows a configuration of a power converter according to thefourth embodiment of the present invention. This power converter (700)is a matrix converter in which bidirectional switches (710) using aswitching element are arranged at nine combinable contact points betweenthe lines from a three-phase a.c. power supply (30) and the stator linesof a three-phase a.c. motor (20), and directly converts the input a.c.voltage into an output a.c. voltage without once converting the inputinto d.c. voltage. Since a matrix converter includes a lower number ofconducting elements, it is theoretically possible to make it compact andhighly efficient. Because the switching element (710) used for thematrix converter needs to be bidirectionally conductive, in the presentembodiment the switching element (710) is configured by pointing two SiCMOSFETs (130) in opposite directions and connecting in series, as shownin FIG. 7 (b).

In each of the above embodiments, SiC MOSFET was illustrated as oneexample of a unipolar device using a wideband gap semiconductor.However, a similar configuration may be considered with unipolar devicesusing another wideband gap semiconductor such as GaN, diamond or thelike.

INDUSTRIAL APPLICABILITY

As described heretofore, a power converter according to the presentinvention is effective in being applied to an air condition or the likein which the efficiency under light load conditions is more importantthan the efficiency in the rated load condition.

The invention claimed is:
 1. A power converter configured so as toperform synchronous rectification by a switching element, wherein theswitching element is comprised of a unipolar device using a wideband gapsemiconductor and including a body diode, the body diode of the unipolardevice is used as a freewheeling diode to circulate the magnetic energyof an inductance, without the need of providing a separate externalfreewheeling diode connected in parallel to the unipolar device, theunipolar device turns on when a reverse current flows through the bodydiode used as the freewheeling diode, so that the reverse current flowsthrough the unipolar device side to thereby perform synchronousrectification, power is converted from direct current to alternatingcurrent, or from alternating current to alternating current, the bodydiode turns on if a voltage is increased up to a predetermined amount ormore when synchronous rectification is carried out, a turn-on voltage ofthe body diode is about 3V and higher than an ON voltage of the unipolardevice, and SiC is used as the wideband gap semiconductor.
 2. The powerconverter of claim 1, wherein the power converter is used for an airconditioner.
 3. The power converter of claim 2, wherein a relationshipbetween an effective current value (I_(rms)) and an on-resistance(R_(on)) of the switching element under an intermediate load conditionof heating operation of the air conditioner, satisfies:I_(rms)<0.9/R_(on).
 4. The power converter of claim 1, wherein theunipolar device is a MOSFET.
 5. The power converter of claim 1, wherein,of an inverter, converter, matrix converter and boosting chopper circuitwhich are constructed to perform synchronous rectification by theswitching element, at least one is included.
 6. The power converter ofclaim 1, wherein a turn-on voltage of the body diode of the unipolardevice is about 3 volts.